In order to meet increasingly stringent federal regulations of NOx and other undesirable emissions, engineers are constantly seeking new strategies of reducing the undesirable emissions. One method of reducing NOx emissions is an urea treatment for exhaust. Although the urea treatment reduces the NOx in the exhaust to a harmless gas, ammonia (NH3) emissions can be created by the reduction, thus potentially requiring an after treatment. Further, the infrastructure to support urea treatment technology, such as urea tanks at gas stations, is not yet available in the United States.
Another method of reducing NOx, without the need for the after treatment and the extensive support infrastructure, is the NOx adsorber catalytic converter, otherwise known as the NOx trap. The NOx trap operates in two alternative phases: a storage phase and a regeneration phase. During the storage phase, the normal operation of an engine produces a reductant-lean exhaust in which the NOx is oxidized and stored on a catalyst, referred to as a NOx adsorber. The storage phase can last anywhere from thirty seconds to ten minutes. During the regeneration phase, the engine produces a reductant-rich exhaust, in which the NOx is de-stored and converted into harmless gasses. The regeneration phase generally lasts one to fifteen seconds. In order to create the reductant-rich environment for the regeneration of the NOx adsorber, additional fuel is required. Because the NOx traps often use fixed lean/rich cycle times to alternate between the two phases, the regeneration phase may last longer than necessary, resulting in a fuel penalty. Moreover, if the regeneration phase continues beyond the de-storage of the NOx adsorber, reductant and ammonia emissions can occur.
In order to lessen the fuel penalty without prematurely stopping the regeneration, a method of determining when the regeneration of the NOx adsorber is completed was suggested in an article, Coupling of a NOx-trap and a DPF for Emission Reduction of a 6-Cylinder HD Engine, published by Renault and presented at the International Motor Symposium, Vienna, Austria, May 15-16, 2003. The Renault Trucks article suggests using an oxygen sensor downstream from the NOx trap to determine the time required to regenerate the NOx adsorber. The oxygen sensor measures an equivalence ratio of the exhaust downstream from the NOx adsorber. The equivalence ratio—the inverse of lambda—is defined as stoichiometric air-to-fuel ratio divided by actual air-to-fuel ratio. When the equivalence ratio of the downstream exhaust exceeds one, the NOx adsorber is regenerated.
Because the oxygen sensor method can determine the end of regeneration based on the reactions occurring during each NOx, adsorber cycle, the use of the oxygen sensor may be a more accurate method to determine the end of regeneration than the one size fits all timed fuel/lean cycles. However, there is still room for improvement. It is always a goal to further minimize the amount of undesirable emissions, such as NOx, CO and ammonia emissions (NH3) in the exhaust.
The present disclosure is directed at overcoming one or more of the problems set forth above.